The muffled perception of one's own voice is still a major problem with hearing aids. This effect occurs when the ear canal is completely blocked, which is why it is referred to as the occlusion effect (OE). Such blocking of the ear canal occurs especially with hearing aids, which usually consist of a central unit fitted behind the ear and an associated internal unit in the form of an earbud inserted into the auditory canal and blocking it tightly.
The external unit generally comprises a power source in the form of one or more batteries, one or more external microphones, and a processor for processing and possibly amplifying the signal recorded by the external microphone, and an interface to the internal unit that, in turn, has a speaker to which an output signal processed by the processor is fed, which output signal corresponds at best to the natural external sounds at the ear that are recorded by the external microphone so that the wearer of the hearing aid can perceive these natural external sounds at a pleasant volume, without distortion, and in good quality despite impaired hearing. However, hearing aids are also known in which the external microphone is part of the internal unit or in which the components of the external and internal unit form a single compact internal unit.
The muffled perception of one's own voice essentially results from two factors. First, the perception of one's own voice is always a combination of two main signals with respect to the human ear itself. The first main signal is characterized by an acoustic wave component, x′AC(t), that is conducted via the air (AC, Air-Conducted), and the second main signal is characterized by an internal component, x′BC(t), that is conducted via the bone and cartilage (BC, Bone-Conducted), as shown in FIG. 1. Thus, one hears one's own voice in the ear from two sources, from the airborne sound x′AC(t) and from the structure-borne sound x′BC(t). This is also the reason why one hears one's own voice differently when speaking than when one hears oneself from a recording. After all, the structure-borne sound component x′BC(t) is missing from the recording. Second, the internal part of a hearing aid, i.e. the earbud, blocks the auditory canal and thus alters its acoustic terminating resistance. The internal part also poses an obstacle to acoustic waves from outside the ear that damps the high frequencies of the airborne sound signal x′AC(t). Moreover, the low-frequency components introduced into the auditory canal by the structure-borne sound signal x′BC(t) cannot escape the auditory canal. This leads to an amplification of the low frequencies by up to 30 dB in extreme cases.
Mechanical solutions for preventing the occlusion effect are known and include the ventilation of the ear canal or a deep insertion of the hearing aid into the ear canal, for example (see Thomas Zurbrugg, “Active Control Mitigating the Ear Canal Occlusion Effect caused by Hearing Aids,” Ph.D. dissertation, EPFL Lausanne, Lausanne, 2014). However, these are not without drawbacks. For instance, ventilation through a vent opening in the earbud increases feedback between the outside microphone and the speaker. Furthermore, deep insertion of the earbud into the auditory canal adversely affects wearing comfort.
As an alternative to mechanical compensation for the occlusion effect, approaches have therefore been developed that employ active noise cancellation (ANC) in order to achieve “electronic ventilation.” In these approaches, a second microphone is used that is located next to the speaker in the internal unit/earbud and records the acoustic signals in the auditory canal, with the recorded signal being fed back negatively to the signal to be emitted by the speaker, and with a controller arranged in the feedback branch having the task of influencing the signal to be outputted by the speaker such that the occlusion effect is minimized.
Such an approach is described, for example, in the above-described publication by Thomas Zurbrugg, in European patent application EP 2 640 095 [U.S. Pat. No. 9,319,814], and in international patent application WO 2006/037156 [U.S. Pat. No. 8,116,489], as well as in the publications “Active cancellation of occlusion: An electronic vent for hearing aids and hearing protectors,” Journal of the Acoustical Society of America, vol. 124, No. 1, pp. 235-240, 2008 and M. Sunohara, M. Osawa, T. Hashiura, and M. Tateno, “Occlusion reduction system for hearing aids with an improved transducer and associated algorithm,” in 2015 23rd European Signal Processing Conference (EUSIPCO), 2015, pp. 285-289. However, this prior art uses a fixed, i.e. immutable, controller. The occlusion effect is however different for each person due to the shape and length of their auditory canal and in each application, since a user does not always insert the internal unit the same way into the auditory canal. Thus, both the orientation of the earbud/angle of the speaker and the insertion depth of the internal unit vary with each use. The use of a fixed controller in the individual user therefore does not lead to a good result.
In addition, solutions with adaptive controllers that have to be manually set or parameterized for a specific user are known from the publications “R. Borges, M. Costa, J. Cordioli, and L. Assuiti, “An Adaptive Occlusion Cancers for Hearing Aids,” in IEEE Workshop on Signal Processing to Audio and Acoustics, 2013, and M. Sunohara, K. Watanuki, and M. Tateno, “Occlusion reduction system for hearing aids using active noise control technique,” Acoustical Science and Technology, Vol. 35, No. 6, pp. 318-320, 2014. It is true that an adaptive controller does lead to an improvement in the suppression of the occlusion effect given the individual adaptation. With respect to the various applications in terms of orientation and insertion depth of the internal unit, the previous approaches do not lead to satisfactory results. In particular, the stability of the overall system with the feedback controller is not considered in the literature but represents one of the main problems of the electronic reduction of the occlusion effect.